It is known that a cold-rolled steel strip having a high drawing quality can be produced by tightly or loosely coiling a cold-rolled steel strip and, then, by annealing it batchwise in a box type annealing furnace. This type of method needs several days to complete the entire process thereof and, therefore, is extremely inefficient. In order to avoid the above-mentioned disadvantage, various attempts have been made to continuously carry out the annealing process, and some of the attempts have been practicably used in industry.
The continuous annealing method can exhibit an extremely high efficiency in comparison with the conventional batch type annealing method. However, it is strongly desired to increase the efficiency of the continuous annealing method to such an extent that the continuous annealing operation is completed within a few minutes.
In a known continuous annealing process, a steel strip is heated in a reducing atmosphere. In this case, the heating operation is effected by using an electric heater or a radiation heating tube in which a fuel is burnt. However, this indirect heating of the steel strip by the radiation heating tube causes the heating rate and heat efficiency to be poor, and also, requires a large heating device and a long time to complete the annealing operation.
In order to accelerate the continuous annealing operation, it has been attempted to rapidly heat the steel strip by using a direct fired furnace or to rapidly cool the heated steel strip with water or a mixture of gas and water in the initial stage of the cooling operation. Such a rapid heating method also allows elimination of an electrolytic cleaning operation before the rapid heating operation. However, both the rapid heating operation and the rapid cooling operation in the above-mentioned processes cause an oxide layer to be formed on the peripheral surface of the steel strip. Therefore, it is necessary to eliminate the oxide layer from the annealed steel strip. Examples of the accelerated continuous annealing methods are as follows.
(1) Japanese Patent Application Laying-open (Kokai) No. 52-14431 (1977) discloses an annealing process in which a steel strip is rapidly heated to a predetermined temperature and maintained at the temperature in a direct fired furnace and, then, rapidly cooled with water, reheated, overaged and, finally, subjected to an acid pickling operation to remove an oxide layer formed on the peripheral surface of the steel strip.
(2) Japanese Patent Application Laying-open (Kokai) No. 53-17518 (1978) discloses a process wherein a steel strip is rapidly heated to a predetermined temperature and maintained at the temperature in the direct fired furnace, rapidly cooled with water and, overaged while the oxide layer on the peripheral surface thereof is removed by reducing it.
Especially, in the above-mentioned process (1) the heating and cooling operations result in the formation of a considerably large thickness of the oxide layer, and this large thickness causes the time necessary for completing the elimination of the oxide layer to be undesirably long. Also, in the process (1), in order to overage the steel strip after the rapid cooling, it is necessary to reheat the steel strip to an overaging temperature thereof.
In the above-mentioned process (2), the elimination of the oxide layer from the steel strip is carried out by the overaging operation at a relatively low temperature. Therefore, in order to effectively attain the elimination of the oxide layer, the reducing operation should be carried out by using a strictly controlled reducing atmosphere having a special concentration of hydrogen and a specified dew point.
Usually, the cold-rolled low carbon steel strip is subjected, after the annealing operation, to a surface processing, for example, metal plating or coating. Accordingly, it is necessary that, after the annealing operation, the steel strip have a clean peripheral surface suitable for the surface processing.
When an oxide layer having a too large thickness is formed on the peripheral surface of the steel strip during the annealing process, this oxide causes the surface layer to become porous even after the oxide layer is completely reduced. This porous surface exhibits poor surface processing properties, that is, a poor activity of accepting various chemical treatments, a poor binding property to a coating, a poor resistance to corrosion even after the surface-processing and a poor plating property.
For example, U.S. Pat. No. 4,140,552 discloses a method for the treatment of an aluminium-killed and low alloy steel strip and sheet surface in a sulfur-bearing atmosphere. In this method, the steel strip is heated to a temperature of from 427.degree. C. to 705.degree. C. in a gaseous combustion product atmosphere from a fuel containing sulfur, to form a sulfur and oxygen rich film on the surface of the steel strip. The sulfur and oxygen-containing film is reduced in a reducing atmosphere containing at least 10% of hydrogen gas and having a dew point of 20.degree. C. or less.
However, the reduction of the sulfur-and-oxygen-containing film on the surface of the steel strip results in the formation of a reduced porous layer. Such a steel strip having a porous surface layer is usable for the hot galvanizing process, but is useless for the electroplating process.
Japanese Patent Application Laid-open No. 53-17518 (1978) discloses a process for annealing a steel strip. In this process, a steel strip is preheated to a temperature of 300.degree. to 400.degree. C. in an exhaust gas containing oxygen; the preheated steel strip is heated in a direct fired furnace, in which a fuel is burnt at a combustion air ratio of 0.9 to 1.0, to a temperature of 700.degree. to 750.degree. C. and maintained at this temperature for a predetermined time period; the heated steel strip is cooled and then overaged at a temperature of 400.degree. to 500.degree. C. in a reducing atmosphere, while causing an oxide layer on the steel strip surface to be reduced and eliminated.
However, in this process, the thickness of the oxide layer formed on the surface of the steel strip is undesirably large. That is, in order to completely reduce the oxide layer, it is necessary that the steel strip stay in the reducing atmosphere for a long time period, even if the concentration of the hydrogen gas in the reducing atmosphere is increased. Also, even if the reduction of the oxide layer is completed, the resultant surface layer of the steel strip is porous, and, therefore, is not suitable for the electroplating process.
Accordingly, it is strongly desired to be able to effect the continuous annealing process to the cold-rolled low carbon steel strip without forming a thick oxide layer on the peripheral surface of the steel strip, and to be able to easily eliminate the oxide layer from the steel strip.